Michael S. Hedrick

The effects of erythrocythemia on blood viscosity, maximal systemic oxygen transport capacity and maximal rates of oxygen consumption in an amphibian.

Summary1.Graded erythrocythemia was induced by isovolemic loading of packed red blood cells in the toad,Bufo marinus. Blood viscosity, hematocrit, hemoglobin concentration, maximal aortic blood flow rate and maximal rates of oxygen consumption were determined after each load.2.Blood viscosity was related to hematocrit in the expected exponential manner; ln η=0.43+0.035 Hct (Fig. 2).3.Maximal blood flow rates in the dorsal aorta were inversely proportional to blood viscosity and fit predictions of the Poiseuille-Hagen flow formula (Fig. 3). The effect of increased blood viscosity was to reduce aortic pulse volume, but not maximal heart rate (Figs. 4, 5).4.Maximal systemic oxygen transport capacity (aortic blood flow rate x hemoglobin concentration x O2 binding capacity of hemoglobin) was linearly correlated with the maximal rate of oxygen consumption (Fig. 6).5.These data indicate that optimal hematocrit theory is applicable for maximal blood flow rates in vivo, and that systemic oxygen transport is the primary limitation to aerial

Control and interaction of the cardiovascular and respiratory systems in anuran amphibians.

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Nitric oxide as a modulator of central respiratory rhythm in the isolated brainstem of the bullfrog (Rana catesbeiana).

max in amphibians.

Nitric oxide modulates respiratory-related neural activity in the isolated brainstem of the bullfrog

The inaugural Kjell Johansen Lecture in the Zoophysiology Department of Aarhus University (Aarhus, Denmark) afforded the opportunity for a focused workshop comprising comparative cardiovascular physiologists to ponder some of the key unanswered questions in the field. Discussions were centred around three themes. The first considered function of the vertebrate heart in its various forms in extant vertebrates, with particular focus on the role of intracardiac shunts, the trabecular (‘spongy’) nature of the ventricle in many vertebrates, coronary blood supply and the building plan of the heart as revealed by molecular approaches. The second theme involved the key unanswered questions in the control of the cardiovascular system, emphasizing autonomic control, hypoxic vasoconstriction and developmental plasticity in cardiovascular control. The final theme involved poorly understood aspects of the interaction of the cardiovascular system with the lymphatic, renal and digestive systems. Having posed key questions around these three themes, it is increasingly clear that an abundance of new analytical tools and approaches will allow us to learn much about vertebrate cardiovascular systems in the coming years.

EFFECTS OF TEMPERATURE AND PHYSICAL ACTIVITY ON BLOOD FLOW SHUNTS AND INTRACARDIAC MIXING IN THE TOAD BUFO MARINUS

In anuran amphibians, respiratory rhythm is generated within the central nervous system (CNS) and is modulated by chemo- and mechanoreceptors located in the vascular system and within the CNS. The site for central respiratory rhythmogenesis and the role of various neurotransmitters and neuromodulators is described. Ventilatory air flow is generated by a positive pressure, buccal force pump driven by efferent motor output from cranial nerves. The vagus (cranial nerve X) also controls heart rate and pulmocutaneous arterial resistance that, in turn, affect cardiac shunts within the undivided anuran ventricle; however, little is known about the control of central vagal motor outflow to the heart and pulmocutaneous artery. Anatomical evidence indicates a close proximity of the centers responsible for respiratory rhythmogenesis and the vagal motoneurons involved in cardiovascular regulation. Furthermore, anurans in which phasic feedback from chemo- and mechanoreceptors is prevented by artificial ventilation exhibit cardiorespiratory interactions that appear similar to those of conscious animals. These observations indicate interactions between respiratory and cardiovascular centers within the CNS. Thus, like mammals and other air-breathing vertebrates, the cardio-respiratory interactions in anurans result from both feedback and feed-forward mechanisms.

Lymph Pools in the Basement, Sump Pumps in the Attic: The Anuran Dilemma for Lymph Movement

Maximal aerobic metabolic rates (MMR) in vertebrates are supported by increased conductive and diffusive fluxes of O2 from the environment to the mitochondria necessitating concomitant increases in CO2 efflux. A question that has received much attention has been which step, respiratory or cardiovascular, provides the principal rate limitation to gas flux at MMR? Limitation analyses have principally focused on O2 fluxes, though the excess capacity of the lung for O2 ventilation and diffusion remains unexplained except as a safety factor. Analyses of MMR normally rely upon allometry and temperature to define these factors, but cannot account for much of the variation and often have narrow phylogenetic breadth. The unique aspect of our comparative approach was to use an interclass meta-analysis to examine cardio-respiratory variables during the increase from resting metabolic rate to MMR among vertebrates from fish to mammals, independent of allometry and phylogeny. Common patterns at MMR indicate universal principles governing O2 and CO2 transport in vertebrate cardiovascular and respiratory systems, despite the varied modes of activities (swimming, running, flying), different cardio-respiratory architecture, and vastly different rates of metabolism (endothermy vs. ectothermy). Our meta-analysis supports previous studies indicating a cardiovascular limit to maximal O2 transport and also implicates a respiratory system limit to maximal CO2 efflux, especially in ectotherms. Thus, natural selection would operate on the respiratory system to enhance maximal CO2 excretion and the cardiovascular system to enhance maximal O2 uptake. This provides a possible evolutionary explanation for the conundrum of why the respiratory system appears functionally over-designed from an O2 perspective, a unique insight from previous work focused solely on O2 fluxes. The results suggest a common gas transport blueprint, or Bauplan, in the vertebrate clade.

Cardiovascular responses to hypoxia and anaemia in the toad Bufo marinus.

Nitric oxide (NO) is a unique interneuronal neurotransmitter and/or neuromodulator that is involved in a variety of physiological functions within the central nervous system (CNS). In neural tissue, NO is generated from an oxygen-dependent, constitutive NO synthase (NOS) by glutamatergic stimulation of N-methyl-D-aspartate (NMDA) receptors. Recent studies indicate that NO has excitatory effects on breathing within the CNS and mediates a central component of the hypoxic ventilatory reflex in mammals. Because NMDA receptors are important in central respiratory rhythmogenesis, we hypothesized that NO would have significant effects on the central pattern generator (CPG) for breathing in the brainstem. To test this hypothesis, the effects of NO on respiratory-related neural activity were investigated using an in vitro brainstem preparation from North American bullfrogs (Rana catesbeiana). Extracellular recordings of respiratory-related burst activity were made from cranial nerves V, X and XII before and during superfusion of the brainstem with NO-generating compounds, or inhibitors of NO synthesis. Addition of the NO donor, sodium nitroprusside (SNP; 0.1-1.0 mM), or the amino acid precursor for NO synthesis, L-arginine (L-Arg; 0.01-1.0 mM), caused significant increases in respiratory-related burst frequency. Inhibition of NOS with N omega-nitro-L-arginine (L-NA; 5-10 mM), a non-selective NOS inhibitor, caused a significant reduction in burst frequency or reversibly abolished neural activity. Brainstem perfusion with the specific neuronal NOS (nNOS) inhibitor, 7-nitro indazole (7-NI), produced significant, dose-dependent reversible reductions in burst frequency at concentrations of 0.1, 0.5 and 1.0 mM. These results suggest that production of NO, probably via nNOS, provides an excitatory input to the respiratory CPG in the amphibian brainstem. Our results suggest that NO may be a necessary inter- or intracellular messenger for neurotransmission and/or neuromodulation of central respiratory drive to motor effectors in the bullfrog.

Carotid body noradrenergic sensitivity in ventilatory acclimatization to hypoxia

The effects of nitric oxide (NO) on respiratory-related neural activity were investigated using the isolated brainstem preparation from bullfrogs (Rana catesbeiana). Addition of the NO donor, sodium nitroprusside (SNP), or the amino acid precursor for NO synthesis, L-arginine (L-Arg), produced significant increases in respiratory-related burst frequency. Inhibition of nitric oxide synthase (NOS) with N(omega)-nitro-L-arginine (L-NA), a non-selective NOS inhibitor, 7-nitro indazole (7-NI), reversibly abolished burst activity. These results suggest that production of NO, probably via neuronal NOS (nNOS), provides a facilitatory input to the respiratory central pattern generator (CPG) in the amphibian brainstem. Endogenous production of NO may be a necessary inter- or intracellular messenger for neurotransmission and/or neuromodulation of central respiratory drive to motor effectors in the bullfrog.